In the conventional art of air conditioners that are capable of heating operation, an air conditioner that comprises a refrigerant heating function for the purpose of increasing heating capacity has been proposed. For example, in an air conditioner according to Japanese Laid-open Patent Application Publication No. H06-26696, a refrigerant that flows through a refrigerant heater, which functions as an evaporator, is heated by a burner during heating operation. Here, in the air conditioner recited in Japanese Laid-open Patent Application Publication No. H06-26696, the amount of fuel that the burner burns is controlled during heating operation in accordance with a temperature difference between the temperature of the refrigerant on the inlet side of the refrigerant heater, which functions as an evaporator, and the temperature of the refrigerant on the outlet side of the refrigerant heater.